Originally Posted by
PaulH1
High speed and high altitude aircraft possess an abundance of both Kinetic and Potential energy. As profile drag increases as the square of IAS, then increasing drag (Speedbrakes, etc) at high speed will destroy more energy than it will at lower speeds. Slowing the aircraft down and then using speed brakes will not have as much effect as using the speed brakes at high speed before reducing.
Actually that explenation may not be 100% correct....
Yes, drag increases with the square of IAS, but so does lift. And lift always (+/- a few percent) has to equal weigt, hence you can not vary drag by selecting speed that easily.
However, speed brakes produde the same delta in CD at any speed, but the lower the CL the higher the impact on L/D, and with L being constant this is what finally gives you drag. So at high speed with low CL, the effect of a delta CD is higher than at high CL, which in the end means the same: at high speed and low CL speed brakes are most powerful.
You may however not be allowed to fully deploy them at high speed (or hydraulics may not be able to fully deploy them against the high loads)
Can somebody do the quick math how steep an A320 can dive at the maximum speed at which full spoilers are allowed? More or less than the -15° nose down flight path the envelope protection does allow?
(15° nose down however feels like 45° the first time a non-aerobatic pilot tries it...)